1. Field of Invention
The present invention relates in general to the field of electronic packaging. More particularly, the present invention relates to electronic packaging that provides carbon dioxide gettering for a chip module assembly.
2. Background Art
Electronic components, such as microprocessors and integrated circuits, are generally packaged using electronic packages (i.e., modules) that include a module substrate to which one or more electronic component(s) is/are electronically connected. A single-chip module (SCM) contains a single electronic component such as a central processor unit (CPU), memory, application-specific integrated circuit (ASIC) or other integrated circuit. A multi-chip module (MCM), on the other hand, contains two or more such electronic components.
Generally, each of these electronic components takes the form of a flip-chip, which is a semiconductor chip or die having an array of spaced-apart terminals or pads on its base to provide base-down mounting of the flip-chip to the module substrate. The module substrate is typically a ceramic carrier or other conductor-carrying substrate.
Controlled collapse chip connection (C4) solder joints (also referred to as “solder bumps”) are typically used to electrically connect the terminals or pads on the base of the flip-chip with corresponding terminals or pads on the module substrate. C4 solder joints are disposed on the base of the flip-chip in an array of minute solder balls (e.g., on the order of 100 μm diameter and 200 μm pitch). The solder balls, which are typically lead (Pb)-containing solder, are reflowed to join (i.e., electrically and mechanically) the terminals or pads on the base of the flip-chip with corresponding terminals or pads on the module substrate.
Typically, a non-conductive polymer underfill is disposed in the space between the base of the flip-chip and the module substrate and encapsulates the C4 solder joints. The C4 solder joints are embedded in this polymeric underfill and are thus protected from corrosion caused by moisture and carbon dioxide in the air. However, as discussed below, the use of the polymeric chip underfill disadvantageously renders the assembled flip-chip(s)/module substrate un-reworkable.
FIG. 1 illustrates an example of a conventional multi-chip module assembly 100 that utilizes C4 solder joints and a polymeric chip underfill. FIG. 2 is an enlarged view of the C4 solder joints and the polymeric chip underfill of the conventional multi-chip module assembly 100. In many computer and other electronic circuit structures, an electronic module is electrically connected to a printed circuit board (PCB). For example, the conventional multi-chip module assembly 100 shown in FIGS. 1 and 2 includes capped module 105 electrically connected to a PCB 110. Generally, in connecting an electronic module to a PCB, a plurality of individual electrical contacts on the base of the electronic module must be connected to a plurality of corresponding individual electrical contacts on the PCB. Various technologies well known in the art are used to electrically connect the set of contacts on the PCB and the electronic module contacts. These technologies include land grid array (LGA), ball grid array (BGA), column grid array (CGA), pin grid array (PGA), and the like. In the illustrative example shown in FIG. 1, a LGA 115 electrically connects PCB 110 to a module substrate 120. LGA 115 may comprise, for example, conductive elements 116, such as fuzz buttons, retained in a non-conductive interposer 117.
In some cases, the module includes a cap (i.e., a capped module) which seals the electronic component(s) within the module. The module 105 shown in FIG. 1 is a capped module. In other cases, the module does not include a cap (i.e., a bare die module). In the case of a capped module, a heat sink is typically attached with a thermal interface between a bottom surface of the heat sink and a top surface of the cap, and another thermal interface between a bottom surface of the cap and a top surface of the electronic component(s). For example, as shown in FIG. 1, a heat sink 150 is attached with a thermal interface 155 between a bottom surface of heat sink 150 and a top surface of a cap 160, and another thermal interface 165 between a bottom surface of cap 160 and a top surface of each flip-chip 170. In addition, a heat spreader (not shown) may be attached to the top surface of each flip-chip 170 to expand the surface area of thermal interface 165 relative to the surface area of the flip-chip 170. The heat spreader, which is typically made of a highly thermally conductive material such as SiC, is typically adhered to the top surface of the flip-chip 170 with a thermally-conductive adhesive. Typically, a sealant 166 (e.g., a silicone adhesive such as Sylgard 577) is applied between cap 160 and module substrate 120 to seal the chip cavity 167. In the case of a bare die module, a heat sink is typically attached with a thermal interface between a bottom surface of the heat sink and a top surface of the electronic component(s). Heat sinks are attached to modules using a variety of attachment mechanisms, such as adhesives, clips, clamps, screws, bolts, barbed push-pins, load posts, and the like.
Capped module 105 includes a module substrate 120, a plurality of flip-chips 170, LGA 115, and cap 160. In addition, capped module 105 includes C4 solder joints 175 electrically connecting each flip-chip 170 to module substrate 120. As best seen in FIG. 2, capped module 110 also includes a non-conductive polymer underfill 180 which is disposed in the space between the base of each flip-chip 170 and module substrate 120 and encapsulates the C4 solder joints 175. C4 solder joints 175 are embedded in polymeric underfill 180 and, thus, as mentioned above, are protected from moisture and carbon dioxide in the air. Without polymeric chip underfill 180, the solder balls of C4 solder joints 175 would corrode, and electrically short neighboring solder balls. Atmospheric carbon dioxide is the primary factor controlling corrosion of the Pb-containing solder balls of C4 solder joints 175, presumably through a series of reaction steps known as the “Dutch reaction”. The Dutch reaction is initiated by the oxidation of lead in the presence of O2 and H2O to form lead hydroxide. Lead hydroxide and acetic acid react in two steps to form basic lead acetate. Decomposition of basic lead acetate by CO2 regenerates lead acetate and H2O so the reaction can proceed again. The reaction is autocatalytic as long as O2 and CO2 are available. Over time, CO2, O2 and moisture seep into chip cavity 167 (e.g., through sealant 166). Polymeric chip underfill 180 protects C4 solder joints 175 but, unfortunately, renders the assembled flip-chips 170/module substrate 120 un-reworkable. Generally, it is preferable to use technologies that provide reworkability. However, the use of polymeric chip underfill 180 stands as an obstacle to reworkablility and, thus, increases the cost of manufacturing and maintenance.
Two approaches have been proposed to simultaneously address the issue of C4 solder joint corrosion as well as the desire to provide reworkability. FIG. 3 illustrates an example of such an approach in a proposed multi-chip module assembly 300 that utilizes a C-ring seal 301, which is interposed between a module substrate 320 and a cap 360. A non-conductive frame 302 is mounted between PCB 110 and the periphery of module substrate 320. Unfortunately, C-ring seal 301 requires a larger module substrate 320 and a larger cap 360 (compared to module substrate 120 and cap 160 shown in FIG. 1) and, thus, results in the loss of precious PCB real estate (i.e., the larger footprint of module substrate 320 and cap 360 occupies a larger area on PCB 110) as well as increased manufacturing cost.
FIG. 4 illustrates an example of a second approach in a proposed multi-chip module assembly 400 that utilizes a molecular sieve desiccant (MSD) 401. MSD 401 is exposed through a passage 402 to a module cavity 467, which encloses C4 solder joints 175 as well as LGA 115. Passage 402 extends from a recess in heat sink 450, through a thermal interface 455 and a cap 460, and into module cavity 467. MSD 401 is a solid sorbent media, e.g., 5A zeolite available from UOP, LLC (Des Plaines, Ill.). Module cavity 467 is sealed using a rubber gasket 403 seated in a non-conductive frame 404 interposed between cap 460 and PCB 110. Typically, a total of about 200 gm of MSD is provided in one or more cartridges 406 and exposed to module cavity 467 to remove moisture and carbon dioxide therefrom. A drawback to this approach is that the MSD merely absorbs the carbon dioxide and, consequently, has a limited capacity. This is a significant drawback because MSD has relatively low removal efficiency. As shown in FIG. 5, for example, 200 gm of 5A zeolite MSD can hold about 1.6 gm of carbon dioxide at 25° C., assuming dry air contains 0.225 torr carbon dioxide. FIG. 5 is based on a similar figure in Lila M. Mulloth & John E. Finn, “Carbon Dioxide Adsorption on a 5A Zeolite Designed for CO2 Removal in Spacecraft Cabins”, NASA/TM-1998-208752, 1998. Moreover, because the MSD is not specific for carbon dioxide, the relative removal efficiency of carbon dioxide will depend on the relative humidity because moisture is also readily absorbed by the MSD.
Therefore, a need exists for an enhanced method and apparatus for protecting solder joints from corrosion caused by carbon dioxide within the chip cavity of a chip module.